In ink-jet printing methods, droplets of ink are directly projected onto a recording medium from very fine nozzles and allowed to adhere to the recording medium to form characters and images. The ink-jet printing methods have been rapidly spread because of their various advantages such as easiness of full coloration, low costs, capability of using plain paper as the recording medium, non-contact with printed images and characters, etc.
In recent years, ink-jet printers have been extensively used in domestic applications, office applications and commercial printing applications, and there is a current tendency that these ink-jet printers are of a high-speed printing type. In order to realize high-speed printing of the ink-jet printers, there have been adopted the method of increasing a capacity of each ink droplet, and the method of improving a printing head (from a serial head to a line head), etc., to reduce a printing frequency and increase a printing speed.
However, the high-speed printing method which is incapable of dual or triple overprinting unlike the conventional printing methods has many problems to be solved. That is, inks used for the high-speed printing are required to have sufficient dispersion stability and ejection stability so as not to cause deterioration in printing quality such as occurrence of thin-spots or bare spots. In addition, colorants used in the printing ink are required to exhibit a high color intensity such that printed images and characters have a high optical density only by one printing operation. In particular, when printed on a plain paper, the colorants must be prevented from penetrating into the paper.
There has been proposed the method of improving a dispersion stability and a storage stability of a water-based pigment dispersion by using two kinds of polymer dispersants therein.
JP 2000-239392A discloses a process for producing an aqueous pigment dispersion which includes the first step of dispersing a pigment in an aqueous medium using a surfactant and/or a water-soluble resin and the second step of mixing the resulting dispersion with a solution prepared by dissolving a self-emulsifiable resin having a specific molecular weight in a water-soluble solvent to deposit the self-emulsifiable resin on a surface of the pigment, for the purpose of improving a storage stability of the dispersion, etc.
JP 2004-277507A discloses an aqueous dispersion of pigment-enclosing polymer particles having a specific average particle size which includes a hydrophilic polymer, a hydrophobic polymer and a pigment, and a water-based ink containing the aqueous dispersion, for the purpose of improving a gloss of prints, etc.
JP 2003-292838A discloses a composition for ink-jet printing which is produced by dispersing a pigment in water using an organic polymer compound (A) having a specific Tg, a specific acid value and a specific molecular weight as a dispersant, and then adding an organic polymer compound (B) having a specific Tg, a specific acid value and a specific molecular weight to the resulting dispersion, and a recording solution for ink-jet printing, for the purpose of improving a storage stability thereof.
JP H11-116881A discloses a colorant composition containing, as a colorant, capsulated particles obtained by enclosing a pigment in a capsule formed of a hydrophilic resin and a hydrophobic resin, and an ink for ink-jet printing, for the purpose of preventing clogging of nozzles, etc.
JP 2000-219841A discloses an aqueous pigment dispersion using two or more kinds of anionic group-containing organic polymer compounds which are different in solubility parameter from each other, and an aqueous recording solution, for the purpose of attaining satisfactory dispersion stability, dispersion viscosity and printability thereof.
JP 1108-209045A discloses a process for producing an ink for ink-jet printing containing a pigment, a water-soluble resin dispersant, a resin emulsion, a sugar and a water-based dispersing medium in which the pigment is dispersed in the dispersing medium and then mixed with various additives to prepare an ink, and thereafter the obtained ink is allowed to pass through a plurality of fine orifices to suitably control a particle size of the pigment particles therein.
JP 2000-160093A discloses an aqueous pigment dispersion which contains at least a pigment and two or more kinds of anionic group-containing organic polymer compounds which are different in weight-average molecular weight from each other.
JP 1110-195352A (JP 3069543) discloses an ink composition for ink-jet printing which includes a pigment dispersion containing a medium solution containing a water-insoluble hydrosol polymer.
According to the above-described patent documents, the resulting dispersions are improved in dispersion stability to a certain extent, but tend to be deteriorated in printability on plain papers upon high-speed printing and hardly exhibit a high optical density on prints.
JP 2007-92059A (JP 3981396) discloses a process for producing a water-based pigment dispersion in which an emulsion composition (A) containing a water-insoluble polymer, an organic solvent, a neutralizing agent and water is mixed with a pigment (B) to prepare a preliminary dispersion, and then the resulting preliminary dispersion is continuously dispersed and separated using a media disperser and further subjected to dispersing treatment using a homogenizer. However, when using such a water-insoluble polymer, the pigment particles tend to be hardly finely pulverized and dispersed, so that the resulting dispersion fails to exhibit a sufficient optical density required for the high-speed printing.